Quantification and quality control

Quality assurance (QA) and quality control (QC) is important in environmental and analytical chemistry to asses that the quality standards are met and to ensure accuracy and precision in the analytical results [78]. QA is the guarantee that the quality of the analytical results is what is claimed in the basis of the quality control applied in all stages from sampling to analysis. QA is not synonymous with quality control. QA is meant to protect the failures of the quality control and QC is therefore a essential component in QA. QA and QC are thus usually jointly recognised and often refereed to as QA/QC. QC is the maintenance and statement of the quality of the analytical results, specifically that it meets or exceeds some minimum standard based on known testable criteria [78]. The direct involvement of QA in this thesis is limited thus the focus of discussion will be on quality control.

QC is included in all steps from sampling to analysis and includes storage, trans-portation, preservation, preparation and analyses of the samples [79]. QC measures are important for the control of possible contamination when handling samples and from instruments, inferences, cross-contamination between samples, loss of the sample in preparation steps, inaccuracy in quantification etc. These effects can be accounted for through the usage of blanks, standards and replicates as well as validation of instruments [79]. Often due to limitations in time and resources, not all QC measures are possible to include but it is important to audit the effects the different steps have on the sample. Standardization of methods is important for securing the quality of the work, and for reproducibility and comparability of analytical data. The International Organisation of Standardization (ISO) provides guidance and standards for all the steps in management of environmental samples [80].

2.5.1. Sampling

Due to limitations in time and resources, especially for the field work, it is not al-ways possible to follow all of the guidance in the ISO standards. However, basing

2.5. Quantification and quality control

the field work on the standards will increase the quality, help prevent contam-ination and make the samples homogeneous and representative for the purpose [81].

The ISO 5667 series describes all the steps involved in water and sediment sam-pling. The series comprises of standards for designing and planning of the field work (ISO 5667:1) [81], sampling of river water (ISO 5667:6) [82], river sediment (ISO 5667:12) [83], seawater (ISO 5667:9) [84] and marine sediment (ISO 5667:19) [85] and the handling and preservation of water samples (ISO 5667:3) [86] and sediment samples (ISO 5667:15) [87]. ISO 5667:14 provides guidance on quality control and assurance [88].

Before sampling, it is important to plan the sampling so that the samples will be representative for the area and the objective wished investigated. When sam-pling for metal analysis, metal free equipment is important to use for samsam-pling and handling to avoid contamination. Replicates for each sample location will increase the reliability of the analytical result [81]. For water sampling, the equip-ment should be rinsed with the sampling matrix at least three times to avoid cross-contamination. Preservation of water samples with acid, often nitric acid, and storage in low temperature in either refrigerator or freezer are important to prevent biological activity and chemical reactions in the sample [82, 84]. Labeling of samples and notes on exact sample location (e.g. GPS-coordinates and depth) are recommended to avoid mix-ups and for reporting and further monitoring of the site [81].

2.5.2. Blanks

Blanks account for interference by other species in the sample and for trace of analyte found in reagents used for sample preservation, preparation and analy-sis. Frequent measurement of blanks also detects whether analyte from previous samples are carried into subsequent analysis by adhering to vessels or instruments [89]. There are three types of blanks; field blank, method blank and reagent blank.

Field blanks are brought out in the field and exposed to the environment that the samples are collected in to trace possible contamination from the sampling process.

Method blanks are taken through all the steps in sample preparation and analysis to trace possible contamination from the preperation steps. A reagent blank is similar to a method blank but has not been subjected to all sample preparation procedures and monitors purity of reagents in analysis [90].

2.5.3. Standards

Standards are used for instrument calibration and instrument verification/accuracy.

A standard is a material or solution containing a known amount of a specific sub-stance(s) [74]. Certified reference material (CRM) is a standard with certified levels of analyte in realistic material similar to the matrix of the sample and is used for testing the efficiency of the sample preparation steps [91]. An external standard or a calibration standard is used for calibration of a analytical tech-nique/instrument. The standard is an analyte or a group of analytes similar to the analyte of interest in the sample and is run separately from the sample in different concentrations. Concentration is then plotted as a function of peak area in a calibration curve. The unknown concentration in the sample is then deter-mined by using the measured peak area and the calibration curve. An internal standard is an analyte or a group of analytes chemically similar to the analytes of interest and are added to the sample at a constant concentration. The internal standard must be an analyte chemically similar but different from the analytes of interest in the sample. A calibration curve is then made by plotting the internal standard peak area to analyte peak area ratio as a function of the known constant concentration [74].

2.5.4. LOD and LOQ

Limit of detection (LOD) is the smallest concentration of analyte in the sample which can be reliably distinguished from zero [92]. LOD is often set to being the concentration which induces signals that is three times higher than the background noise levels. Noise is the change in detector response caused by drift, chemicals, electrical components and other variations in the system without the sample being present. Concentrations over the LOD is not necessarily quantifiable. Limit of quantification (LOQ) is the smallest concentration of analyte in the sample which can be reliably quantified. LOQ is often set to the concentration which induces signals that are ten times higher than the background noise level [92].